1. Field of the Invention
The present invention relates to a CCD (Charge Coupled Device), and more particularly, to a structure of a signal detecting part for a CCD with an improved signal sensitivity.
2. Discussion of the Related Art
In general, a CCD (Charge Coupled Device) is an image sensor which converts an image signal into an electrical signal. It is also called a "CCD image sensor" as the CCD uses a charge coupling in scanning and reading the image signal. The CCD is provided with a photodiode (PD) for converting an image signal into an electrical signal, a VCCD (vertical CCD) for transferring a signal charge converted by the photodiode (PD) in a vertical direction, an HCCD (Horizontal CCD) for transferring the charge transferred from the VCCI) in a horizontal direction, and a sense amplifier (SA) for sensing the signal charge transferred from the HCCD.
In order to improve a sensitivity of the CCI), different methods are suggested. One example is a method for improving a conversion ratio, in which a parasitic capacitance of a signal detecting part is reduced to increase a voltage conversion ratio which can be obtained from a signal charge. Another example is a method for improving a filter factor using a microlens. Because the method for improving a conversion ratio is more fundamental than the method for improving a filter factor of the microlens and the like, the improvement of the conversion ratio should first be considered.
In order to increase a conversion ratio, a parasitic capacitance between a floating diffusion (FD) region and a gate of a transistor in the sense amplifier should be first reduced. Such a parasitic capacitance is dependent on (1) design criteria, such as an FD area and a size of the transistor in the sense amplifier, (2) layout factors, and (3) process conditions. Consequently, in order to minimize a parasitic capacitance of the signal detecting part, different methods for designing a layout are suggested to minimize the amount of overlap between the FD and a metal or polysilicon layer connecting the gate of the transistor in the sense amplifier.
A CCD according to a related art will be explained with reference to the attached drawings. FIG. 1 illustrates an equivalent circuit of a signal detecting part for the CCD according to the related art.
As shown in FIG. 1, the signal detecting part for the CCD according to related art is provided with an N-type BCCD region 12 on a P-well 11 or a P-type substrate. A reset gate 14 is formed over the N-type BCCD region 12 with a gate insulating film 13 therebetween. An output gate 15 is also formed on the N-type BCCD region, spaced apart from the reset gate 14. A floating diffusion region 16 is formed between the reset gate 14 and the output gate 15, and a reset drain region 17 is formed on one side of the reset gate 14 opposite to the floating diffusion region 16. A first transistor M1 is controlled by a voltage detected at the floating diffusion region 16, and a second transistor M2 is connected to the first transistor M1 in series. The first transistor M1 and the second transistor M2 have source followers. In an ideal case, the second transistor M2 becomes a constant current source. Accordingly, an output current is constant when an output voltage Vout is constant. The source of the first transistor M1 always follows a variation of the voltage provided to the gate.
FIG. 2 illustrates a layout of a signal detecting part for a CCD according to the related art. As show in FIG. 2, the layout of the related art signal detecting part shows the gate 21 of the first transistor M1 disposed perpendicular to the floating diffusion region 16. A source region 22 and a drain region 23 are disposed on both sides of the gate 21 of the first transistor M1. A metal layer 24 is disposed on the floating diffusion region 16 for electrical connection between the floating diffusion region 16 and the gate 21 of the first transistor M1. An output gate 15 and a reset gate 14 are disposed on both sides of the floating diffusion region 16. A reset drain 17 is disposed on one side of the reset gate 14 opposite to the floating diffusion region 16.
In the foregoing related art CCD, signal charges generated by incident lights pass through the HCCD and are collected at the floating diffusion region 16. The amount of signal charges collected at the floating diffusion region 16 is Q, providing a voltage V according to Q=C.times.V. When a high voltage is provided to the reset gate 14, the charges at the floating diffusion region 16 are transferred to the reset drain 17, emptying the floating diffusion region 16 of signal charges and permitting reception of new signal charges. However, for a layout in which the reset gate 14 is disposed as close to the floating diffusion region 16 as possible, defining a source region 22 of the first transistor M1 is difficult because of the limited space available.
Therefore, as shown in FIG. 3, a method for designing a layout is suggested, in which the reset gate 14 is disposed farther away from the floating diffusion region 16 for securing an adequate space for defining the source region 22. However, the layout of FIG. 3 has a larger floating diffusion region 16, resulting in a reduction in the conversion ratio.
Therefore, the signal detecting part for a CCD according to the related art has the following problem. Even though different methods arc suggested to reduce a parasitic capacitance of the signal detecting part by changing a layout, there has been a limitation in reducing the parasitic capacitance of the signal detecting part because the related art layout is determined according to a design rule of a process.